Medical diagnostics and industrial inspection require high spatial resolution detection of x-rays transmitted through a body. For example in a typical computed tomography (CT) scanning system, an x-ray source and an x-ray detector array are positioned on opposite sides of the subject and rotated around the subject in fixed relation to each other. In a CT scanning system using a solid scintillator, the scintillator material of a cell or element absorbs x-rays incident on that cell and emits light which is collected by a photodetector for that cell. Thus, the x-rays, the electromagnetic radiation of interest, stimulate or excite the scintillator material, which then emits scintillating radiation, e.g., light. During data collection, each cell or element of the detector array provides an electrical output signal representative of the present light intensity in that cell of the array. These output signals are processed to create an image of the subject in a manner which is well known in the CT scanner art.
It is desirable to absorb substantially all of the incident x-rays in the scintillator material in order to minimize the x-ray dose to which the body must be exposed during the diagnostic or inspection x-ray measurement. In order to collect substantially all of the incident x-rays, the scintillator material must have a thickness in the direction of x-ray travel which is sufficient to stop substantially all of the x-rays. This thickness depends both on the energy of the x-rays and on the x-ray stopping power of the scintillator material.
As the thickness of the scintillator increases, its transparency to the generated light must also increase so that substantially all of the light generated by the x-rays in the scintillator is collected by the photodetector. Collecting all the light generated by the x-rays maximizes overall system efficiency, the signal to noise ratio, and the accuracy with which the quantity of incident stimulating radiation, i.e., x-rays, may be measured. Furthermore, as the scintillator thickness increases, light generated near the scintillator surface opposite the photodiode has a relatively long distance to “spread out”. The “spreading out” of the light means that the light generated by an x-ray impinging upon the scintillator region directly on a particular photodetector cell may not be detected by that particular photodetector cell, but instead by an adjacent cell. Thus, the spatial resolution of the detector is reduced.
One method used to overcome these problems involves making the scintillator out of glass which can be drawn into fiber bundles. Fiber cladding and interspersed dark fibers can reduce the light spread and thereby improve spatial resolution. However, one disadvantage of this material is that due to the amorphous structure, glass scintillators have inherently low efficiencies of energy conversion of the stimulating x-rays to visible light. The efficiency of an x-ray scintillator material is the percentage of the energy of the absorbed x-rays which is generated as light. Glass scintillators also have relatively poor scintillation properties such as afterglow and radiation damage which limits their utility.
Afterglow in an x-ray detecting scintillator is the phenomena that luminescence from the scintillator due to x-ray excitation can still be observed a long time after the x-ray radiation is absorbed by the scintillator. Upon absorbing x-ray radiation the scintillator will emit light where the intensity of the light decays rapidly at an exponential rate. Additionally, the scintillator will emit a lower intensity light where the light intensity decays much more slowly. The more slowly decaying light is termed afterglow.
Radiation damage in an x-ray scintillator material is the characteristic of the scintillator material in which the quantity of light emitted by the scintillator material in response to the stimulating x-ray radiation changes after the material has been exposed to a high radiation dose.